Hepatitis C is a common infection that can lead to chronic hepatitis, cirrhosis, liver failure, and hepatocellular carcinoma. Infection with the hepatitis C virus (HCV) leads to chronic hepatitis in at least 85% of cases. It is the leading reason for liver transplantation, and is responsible for at least 10,000 deaths annually in the United States (Hepatology, 1997, 26 (Suppl. 1), 2S-10S).
Interferon and interferon in combination with ribavirin are used in the U.S. for hepatitis due to HCV. These treatments are associated with improved serum enzyme response in some patients. The remainder are non-responsive to treatment. For responders, a sustained clinical improvement is seen in only a small percentage of patients; the majority of patients relapse upon cessation of treatment. Thus, the effectiveness of therapy for chronic hepatitis C is variable and its cure rate remains low. Moreover, therapy is often associated with considerable side effects.
New therapies and preventatives are clearly needed for infections and diseases caused by the hepatitis C virus.
The hepatitis C virus is a member of the Flaviviridae family. The genome of HCV is positive strand, single stranded linear RNA (Hepatology, 1997, 26 (Suppl. 1), 11S-14S). HCV displays extensive genetic heterogeneity; at least six genotypes and more than 50 subtypes have been identified.
Following infection by HCV, the viral RNA is translated into a polyprotein. This approximately 3,000 residue polyprotein is subsequently cleaved into individual proteins by host peptidases, as well as virally encoded proteases. The HCV genome encodes structural proteins (required for virus assembly) and nonstructural proteins (required for replication). Some of the nonstructural proteins include: NS2, NS3, NS4A, NS4B, NS5A, and NS5B (J. General Virology, 2000, 81, 1631-1648). NS5B is a RNA-dependent RNA polymerase that is essential for viral replication. In positive stranded RNA viruses, such as HCV, RNA is the sole genetic material. Since mammalian host cells ordinarily lack RNA-dependent RNA polymerase activity, the positive stranded RNA viruses encode their own replicative polymerase (NS5B in the case of HCV), which is essential for the production of virion progeny. The inhibition of NS5B activity, therefore, provides an attractive target for HCV drug design.